In the prior art, when mounting assembly devices on a structure, the operator must, in a first operation, measure the actual thickness of the structure to be assembled to select a fastening device with an appropriate length because a difference may exist between a theoretical reference thickness, known by the operator, and an actual thickness, this difference being due to varying manufacturing tolerances that exist in the area of the assembly, specifically in the thickness of the structural elements.
In practice, however, because it is difficult and tedious to measure this thickness, the result is approximate or even incorrect. Additionally, the assembly of the assembly device itself can cause                compression of the elements to be assembled at the location where the assembly device is mounted due to a lack of hardness of an interposed mastic applied between said elements;        an elongation of said device in response to the exerted force, from one side, by a head of said device and, from the other side, by either a nut or a swage collar.        
All of these inaccuracies can easily result in an incorrect selection regarding the length of assembly device and cause a device to be assembled with a wrong length.
In the prior art, to simplify the selection of a device to use for the assembly, an acceptable grip range is defined for each device. For a given theoretical reference thickness, this range is the difference between the actual minimum thickness and the actual maximum thickness of a structure that can be assembled with a given device, i.e. the potential thickness variation range of all elements in the structure. Typically, this grip range is about one or two-sixteenths of an inch. A family of assembly devices therefore comprises several devices of different lengths, the difference in length separating them generally being the grip range, except on an overlap range.
After the assembly of the assembly device selected by the operator, typically a screw or a swage groove fastener, the operator verifies his selection after tightening a bolt or a swage collar on the fastener grooves. There are then four possible outcomes:                the screw does not protrude from the nut, indicating that the assembly device is much too short, in which case it must be completely replaced;        the length of the device seems to be correct to the person who installed it, in which case he does not replace it, even though it is actually a few tenths of a millimeter too short, representing a serious risk in terms of shearing the threads or the swage collar, or loosening the nut;        the tightened nut or swage collar is not at all seated against the structure, indicating that the device is much too long, in which case it must be completely replaced;        the length of the device seems to be correct to the person who installed it, in which case he does not replace the assembly device, even though it is actually too long, i.e. the tightened nut is not seated against the structure but rather abuts one end of the threading of the device, such that the assembly device does not apply any clamping force onto the structure to be assembled.        
If the operator realizes his incorrect selection, there is a significant loss of material and time because he must, in a second operation, dismantle the device that can no longer perform its function in most cases and reassemble, in a third operation, as many times as necessary, another specific assembly device, called “oversize”.
If he does not realize his incorrect selection, this mistake has serious consequences for the strength of the structure being assembled.
The method for determining the device length in the prior art is therefore long, tedious, inaccurate, costly, and dangerous.